Low-viscosity solvent-free reactive systems have acquired an increased commercial significance in the manufacture of plastics in view of present ecological requirements and since they tend to conserve both energy and solvents.
It has been proposed to use molten .epsilon.-caprolactam (m.p. 70.degree. C.) as a solvent for substantially insoluble compounds of relatively high molecular weight such as polymethylene thioureas, optionally together with non-reactive organic hydroxyl-group-free solvents, such as aromatic hydrocarbons, acetone, ethers, esters, tetrahydrofuran and aliphatic halogenated hydrocarbons (see, e.g. German Pat. No. 910,336).
It has also been proposed to use lactam mixtures or associates of water, alcohols, polyalcohols, oximes, amines and diamines for a variety of different applications as non-reactive or reactive solvents, for the production of foams, cellular and non-cellular polyurethane plastics, catalysts, mold release agents, and the like. (Belgian Pat. Nos. 775,907; 776,906; 803,215; 784,735; and 803,214; German Offenlegungsschriften Nos. 2,330,211 and 2,357,191; and U.S. Pat. No. 3,578,444.)
It is known that a variety of different cyclic amides (such as butyrolactam, valerolactam and .epsilon.-caprolactam) form highly viscous salt-like or crystallized salts with concentrated, anhydrous acids (such as sulfuric acid and hydrohalic acid) (cf. Houben-Weyl, Methoden der Organischen Chemie, Vol. 11/2, page 552 (1958). In addition, these products are readily hydrolyzed with aqueous acids at elevated temperature to form the corresponding salts of the amino carboxylic acids (Houben-Weyl, supra, page 565).
In view of (1) the fairly marked tendency of lactams unsubstituted on the nitrogen to hydrolyze in the presence of acids; (2) the high viscosity of the salt-like compounds; and (3) their tendency to form salts through the formation of crystallizing salts of amino carboxylic acids or of salts of any amino polyamides formed, lactam mixtures or associates (e.g., of the type described in Belgian Pat. Nos. 776,906 or 776,907, such as those prepared from 1 mol of .epsilon.-caprolactam and 1 to 4 mols of polyhydroxyl compounds, monoalcohols or water in the presence of inorganic acids or organic mono- or poly-carboxylic acids) had been expected to be unstable mixtures which, in view of their poor storability, excessive viscosity, and corrosion behavior, would give rise to significant disadvantages when used on a commercial scale.